Air storage gas turbines generally comprise a high pressure turbine with an air inlet, a low pressure turbine with a rotor requiring cooling and, in some cases, a vane carrier, together with one high pressure balance piston and one low pressure balance piston located on a common shaft for thrust balancing the high pressure turbine and the low pressure turbine, respectively.
In an installation designed in this manner, the high pressure balance piston and the low pressure balance piston are located adjacent to one another on the common shaft and are separated only by a balance piston intermediate space. The main part of the low pressure auxiliary air, which is required for cooling the low pressure turbine and for operating the low pressure balance piston, is taken from the high pressure balance piston. This air is, therefore, air originating from the high pressure intermediate space of the high pressure turbine which has been throttled by the sealing strips of the high pressure balance piston. In order to provide a sufficiently large quantity of auxiliary air, the labyrinth clearance in the high pressure balance piston must be made correspondingly large.
The arrangement mentioned above is associated with some shortcomings. The level of the auxiliary air pressure is generally dictated by the demands of the low pressure consumers and may, for example, be 13 bar however the pressure of the main pressure air in the high pressure intermediate space can be more than 40 bar (storage pressure). Accordingly, there are large throttling losses at the high pressure balance piston and the sealing strips of the latter are subject to a high aerodynamic load which can lead to vibration damage. In addition, it is absolutely necessary to provide pressure-controlled valves in order to compensate for mass flow changes caused by clearance changes at the high pressure balance piston. These valves are used either for blowing off low pressure auxiliary air or for introducing air from another source into the low pressure auxiliary air system. Also, the throttled air, being hot because the main high pressure air is generally preheated in a recuperator, can create high temperature stresses in the low pressure balance piston because of the convective heat transfer in the balance piston intermediate space.